Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing my new design.

The outermost broken line illustrates the border of the display screen and forms no part of the claimed design. The remaining broken lines shown within the graphical user interface, including the broken lines within the graphs and charts and the broken line text and indicia, form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 